In inkjet printing, minute droplets of ink are ejected in a controlled manner from one or more reservoirs or printing heads through narrow nozzles on to a substrate which is moving relative to the reservoirs. The ejected ink forms an image on the substrate. For high-speed printing, the inks must flow rapidly from the printing heads, and, to ensure that this happens, they must have in use a low viscosity, typically 200 mPas or less at 25° C. although in most applications the viscosity should be 50 mPas or less, and often 25 mPas or less. Typically, when ejected through the nozzles, the ink has a viscosity of less than 25 mPas, preferably 5-15 mPas and ideally 10.5 mPas at the jetting temperature which is often elevated to about 40° C. (the ink might have a much higher viscosity at ambient temperature). The inks must also be resistant to drying or crusting in the reservoirs or nozzles. For these reasons, inkjet inks for application at or near ambient temperatures are commonly formulated to contain a large proportion of a mobile liquid vehicle or solvent such as water or a low-boiling solvent or mixture of solvents.
Another type of inkjet ink contains unsaturated organic compounds, termed monomers, which polymerise by irradiation, commonly with ultraviolet light, in the presence of a photoinitiator. This type of ink has the advantage that it is not necessary to evaporate the liquid phase to dry the print; instead the print is exposed to radiation to cure or harden it, a process which is more rapid than evaporation of solvent at moderate temperatures. In such inkjet inks it is necessary to use monomers possessing a low viscosity.
Inkjet inks for printing coloured images include a colouring agent that is typically selected from dyes and pigments.
Printed images having high gloss are preferred for a number of applications such as photographic printing. Although inkjet inks comprising a dye colourant can be used to print high gloss images on high gloss substrates, such inks are susceptible to fading when exposed to the atmosphere and/or light. Images formed from inks comprising pigment are less susceptible to fading but it can be difficult to achieve images with uniform gloss when using these inks. In other words, it can be difficult to achieve the same levels of gloss in inked areas of the image and “white” areas, or areas that are not inked.
Furthermore, the surface of the printed image, particularly images formed from solvent-based inkjet inks, can have little solvent resistance and be susceptible to scratching and abrasion.
Colourless inks have therefore been developed for printing under or over coloured images in order to improve properties of the printed image such as adhesion to the substrate, scratch resistance, abrasion resistance, hardness, gloss and resistance to fading. Colourless inks can also be printed together with coloured inks, particularly inks comprising pigment, in order to provide uniform gloss for photographic applications.
There exists a need for alternative colourless inks that can be printed using an inkjet printer.